The Duchenne smile is a genuine smile. It just happens. The corners of the mouth rise and there is a natural contraction of muscles around the eyes. This kind of smile will often precede genuine laughter. It is the kind of smile I like to display and to witness.
However, as I am sure you have noticed yourself, many smiles we give out and see are false or pretend smiles. They happen when the brain sends out a signal to the mouth to form the familiar smile shape. Then there is no muscular contraction around the eyes.
Such smiles occur in various social situations. For example, we smile when attending job interviews or being introduced to new people. Such moments can be stressful but the smile attempts to mask that stress. It is as if we associate smiles with the best versions of ourselves. This is why contestants in beauty pageants smile all the time.
When I was fourteen, a cricket ball hit me in the mouth and damaged my front teeth. One prominent tooth had to have the nerve removed and by the time I reached my late teens it was no longer pearly white. In fact, it was turning grey.
I was very conscious of it, probably over-conscious, and developed a way of smiling that was unnatural - keeping my teeth hidden from view. I have noticed other people smiling in that way and the usual reason is similar self-consciousness about teeth. That way of smiling can send out signals that can easily be misinterpreted in a negative way. We all like to see "genuine" smiles.
My grey tooth was fixed a few years ago and it was suddenly a delight to feel confident about my smile.
In my life I have come across people who smile far too much. Not Duchenne smiles but those false or pretend smiles. They can mask so much. Most of these smilers are insecure people who may be under the misapprehension that to be liked or appreciated all you have to do is keep smiling,